In this unit we will learn some concepts like heat, temperature, thermal expansion, thermal energy and phases of matter. Moreoversome misconceptions about heat and temperature will be explained. Since they make confusions in many students’ mind, we give more importance on this subject. In daily life sometimes we use them interchangeably however, in physics they are totally different concepts. How we measure temperature of the substance, how much heat is required for melting the given ice, which matter expands much with the same amount of heat… We will try to answer all these questions in this section. Let’s start with the definitions of heat and temperature.